The present invention relates to an optical device for contactless measurement of distance from a light source. For example, it applies in the industrial field to non-destructive dimensional checking (measurement of levels or of thickness, checking the profile of components, cartography, robot position-fixing).
Among optical devices for contactless distance measurement, the triangulation systems, which are very widespread in the industrial field, form low-cost and easy-to-use devices. The principle of the measurement of the distance from an object to the device is the calculation of the angle at which this object is seen. A point or line of light projected off-axis (that is to say along an axis differing from the optical axis of the system) onto the object are re-imaged onto a plane in the vicinity of that of a detector forming a light spot; in this type of device, the object is regarded as being a point; the position of the spot on the detector, defined, for example, by the centre of gravity of the distribution of the brightness, is proportional to the tangent of the angle at which the projected point or the line is seen. This device carries out no image recognition and does not need to have good resolution. In contrast, it works with optics of low numerical aperture and exhibits a good depth of measurement field. One of the main drawbacks of this method is the off-axis projection by an illumination system of the light source, which has the effect of creating shadow regions which cannot be measured, in particular when the measured object contains high spatial frequencies (that is to say steep slopes). Moreover, the dimensions of such a system increase markedly with the working distance, because of the angle necessary between the transmission channel and the reception channel.
The devices based on the principle of stereoscopy, developed, for example, for motor vehicle applications, are superior. They employ two optical systems associated with two optical detection means separated by a distance L, each optical system forming one image of an object, the distance from the object then being determined by the measurement of the separation xcex between the two images given by the two detection means. In this type of device, the accuracy of measurement of the distance is directly proportional to the distance L, which dictates having two separate detection means sufficiently far apart from one another. That being so, this type of device is bulky and constricting in use, since it requires very high stability as to the relative position of the detectors. In order to gain in accuracy, some of these devices possess improved image-processing and recognition algorithms, allowing them better to assess the separation xcex (see, for example, the patent EP 0 558 026). These devices require optics with high numerical aperture (low focal-length/diameter ratio) in order to have very good optical resolution, and therefore exhibit a shallow depth of measurement field which dictates that they work with field-adapting objective lenses.
Other distance-measuring devices are based on the investigation of the defocusing of the image point of an object point through a main lens when the object point is shifted on either side of a nominal position (see for example the IEEE document xe2x80x9cTransactions on pattern analysis and machine intelligencexe2x80x9d, vol. 14, No 2, Febuary 1992, pages 99-106, Adelson et al.).
The invention proposes a compact distance-measuring optical device, with excellent accuracy, exhibiting a good depth of measurement field and capable of working on the axis of the device. It includes a set of imaging means forming, from one light source, a set of light spots on a detection means, the distance from the source being determined on the basis of the relative position of the spots. These imaging means are positioned in a plane close to the pupillary plane, each means constituting a sub-pupil. In the device according to the invention, the accuracy on the distance depends on the number of light spots. The higher this number, the more accurate the measurement; hence, with sufficiently small imaging means, for example micro-lenses, the device requires only a single detection means, for example a linear array or a matrix of detectors.
More precisely, the invention relates to a device for contactless measurement of distance from a light source including an optical detection means formed by elementary detectors and a set of N imaging means (6), Nxe2x89xa73, characterised in that:
the imaging means make it possible to image the light source on a plane in the vicinity of that of the detection means, thus forming, on the said means, a set of at least three light spots, each spot being spread over at least two elementary detectors;
the device further includes a calculating circuit making it possible, on the basis of the relative positions of at least three light spots, to calculate at least one parameter characteristic of the distance from the light source to the device with an accuracy depending on the number N of light spots.
Advantageously, the calculating circuit may include a calibration table containing at least one calibration parameter the values of which are determined for certain distances from the light source and compares the characteristic parameter with the values of the said calibration parameter. According to one operating mode, the device may further includes a circuit for spatial position-fixing of the light spots on the detection means, determining, for each light spot, its position with respect to a reference origin. The device according to the invention exhibits the advantage of being compact, simple to use, and fast since it employs simple calculating algorithms; it works with imaging means with low numerical aperture since it does not require optical resolution and thus exhibits a great depth of field without it being necessary to use a field-adapting objective lens, even if that were possible. Moreover, such a device can operate on the axis, its large number of imaging means limiting the shadow regions which cannot be measured.